rust-lang/rust · error

epilogue_noreturn for {:?}

Error message

epilogue_noreturn for {:?}

What it means

Thrown by rustc_codegen_cranelift for the noreturn variant of the inline-asm epilogue. For non-returning asm! blocks (e.g. options(noreturn) or asm that ends in an unconditional trap/unreachable), the backend emits an architecture-specific trap instruction (ud2 on x86_64, brk on aarch64, ebreak on riscv64). Any other InlineAsmArch panics via `_ => unimplemented!("epilogue_noreturn for {:?}", arch)`.

Source

Thrown at compiler/rustc_codegen_cranelift/src/inline_asm.rs:723

                generated_asm.push_str("    addi sp, sp, 16\n");
                generated_asm.push_str("    ret\n");
            }
            _ => unimplemented!("epilogue for {:?}", arch),
        }
    }

    fn epilogue_noreturn(generated_asm: &mut String, arch: InlineAsmArch) {
        match arch {
            InlineAsmArch::X86_64 => {
                generated_asm.push_str("    ud2\n");
            }
            InlineAsmArch::AArch64 => {
                generated_asm.push_str("    brk     #0x1\n");
            }
            InlineAsmArch::RiscV64 => {
                generated_asm.push_str("    ebreak\n");
            }
            _ => unimplemented!("epilogue_noreturn for {:?}", arch),
        }
    }

    fn save_register(
        generated_asm: &mut String,
        arch: InlineAsmArch,
        reg: InlineAsmReg,
        offset: Size,
    ) {
        match arch {
            InlineAsmArch::X86_64 => {
                match reg {
                    InlineAsmReg::X86(reg)
                        if matches!(
                            reg.reg_class(),
                            X86InlineAsmRegClass::xmm_reg
                                | X86InlineAsmRegClass::ymm_reg
                                | X86InlineAsmRegClass::zmm_reg

View on GitHub (pinned to 22057b88b0)

Solutions

1. Use the LLVM backend for the unsupported target.
2. Restrict the noreturn asm! to x86_64/aarch64/riscv64 via cfg, providing a portable Rust-level trap (core::intrinsics::abort() / core::hint::unreachable_unchecked()) on other arches.
3. Add the missing trap instruction for the arch in epilogue_noreturn() upstream.

Example fix

// before
unsafe fn abort() -> ! { asm!("ud2", options(noreturn)); }

// after - portable across arches, no inline asm needed
unsafe fn abort() -> ! { core::intrinsics::abort(); }

// Flag `asm!(..., options(noreturn))` — the noreturn epilogue path is
// explicitly unimplemented in cg_clif.
let src = std::fs::read_to_string(&path)?;
if src.contains("asm!") && src.contains("options(noreturn)") {
    return Err("noreturn inline asm not supported by cranelift backend");
}

// Process-abort panic; cannot be caught in user code.
// Mitigate by removing `options(noreturn)` from asm! or compiling with LLVM.

When it happens

Trigger: Compiling an asm! block marked options(noreturn) (or otherwise not returning) on a target whose InlineAsmArch is not X86_64/AArch64/RiscV64 with the Cranelift backend. Common in panic handlers, abort paths, or low-level boot code that uses asm to halt/trap.

Common situations: A #![no_std] binary or kernel using cg_clif that issues a noreturn asm trap (asm!("ud2", options(noreturn))) on an unsupported arch; cross-compiling panic handlers to LoongArch/PowerPC/S390x with the cranelift backend selected.

Related errors

• prologue for {:?}
• epilogue for {:?}
• save_register for {:?}
• restore_register for {:?}
• {:?}

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/6c2d3f4f9c946d6c.json. Report an issue: GitHub.